The dispersion of nanoscopic particles in host polymers has resulted in well-documented improvements in various properties, such as thermo-oxidative stability, response to mechanical load, thermal expansion, space survivability, abrasion resistance, moisture uptake, electrical characteristics, and other surface-related properties. As a result, the application of nanotechnology has enabled an expansion in the end-use envelope of many organic materials. Nanoparticles classified as polyhedral oligomeric silsesquioxanes, or POSS, have proven to be a particularly effective class of additives due to their inherent size, shape, rigidity, and versatility in function. The ability to chemically tailor POSS through synthetic manipulation over its organic periphery offers a unique design tool for controlling particle affinity and resultant dispersion in host materials. The number of peripheral substituents is dependent on the size of the silsesquioxane cage itself, or more precisely, the number of silicon atoms contained within the cage. The nature of those substituents may be depends on the limitations defined by state-of-the-art POSS synthesis methods. There are advantages and disadvantages in the choice of an inert or reactive periphery. In general, inert functionalities do not require an adjustment of polymerization stoichiometry or impose complications arising from differences in the reaction kinetics amongst the involved chemical species; however, inert functionalities provide limited control in the formation of desired material morphologies and increased possibility for nanoparticle flux to a more desired energy state over time that may result in a possible degradation of properties during such a process. Reactive moieties, on the other hand, allow for more precise control over copolymer architecture and impede nanoparticle migration over time (due to covalent attachment to the host); however, reactive moieties stoichiometric balance that is often hindered by steric restraints. A review of the current state of inert and reactive POSS technologies may be found in Fina, A. et al., Journal of Materials Chemistry 2010, 20(42), 9297-9305. Overall, selection of the POSS periphery should be based on the structure of the polymer that is intended to be modified and the desired end-use properties.
To date, an array of POSS molecules have been developed for a variety of applications. While POSS is useful as a commercial additive, there are currently some limitations in its utility. The limitations are especially true in the modification of high temperature polymers, due to a general weakness in the thermal stability of organic peripheries with C—C bonds having dissociation energies in the close proximity of 80 kcal/mol. Currently, the most thermally robust versions of POSS have cages functionalized with complete inert aromaticity. Examples include Octaphenyl™ and Dodecaphenyl™ POSS, which feature pure phenyl moieties comprised of C═C bonds having dissociation energies of approximately 150 kcal/mol. Unfortunately, such molecules demonstrate a propensity to crystallize to the extent that their assembled aggregation is unaffected by shear forces imposed by conventional polymer blending and compounding methods. Crystallization is also to the extent that the exemplary POSS cages degrade during the melting process, both characteristics preventing effective dispersion into host materials. From a structural perspective, POSS molecules readily crystallize due to the rigid and regular nature of the silsesquioxane cage itself. This propensity for crystallization has been shown to be disrupted through the design and synthesis of a heterogeneous organic periphery, which is described in Moore et al., Journal of Organometallic Chemistry 2011, 696(13), 2676-2680. The achievement of a balance between good thermal stability and dispersability lies in molecular design.
To maximize the full benefits of POSS incorporation into polymer hosts, such as in terms of the delivered composite properties, the POSS molecules design requires consideration of molecular assembly during incorporation into a polymer host. For POSS molecules possessing a reactive or partially reactive periphery, geometric forethought of the attachment route with a polymer host network is necessary. For example, POSS may be incorporated through copolymerization by strategic placement of the silsesquioxane cage in the main chain (“bead-on-a-string”) of a growing polymer chain or network, as described in U.S. Pat. No. 6,767,930, issued To Svejda et al. in 2004, or conversely, in a pendant, or dangling-type conformation, such as is described in Wright et al., Macromolecules 2006, 39(14), 4710-4718. The resulting architectures often yield a marked difference in morphology and as a result, in delivered properties.
In terms of imparting reactive functionality to POSS, amine groups offer the most versatility due to their compatibility with many types of polymers, viz. epoxies, cyanate esters, and polyimides (both thermoplastic and thermosetting in nature). Twelve varieties of amino-containing POSS molecules are commercially available, however, all but one of those compounds contain peripheries comprised of thermally labile chemical groups, i.e., any combinations of isobutyl, isooctyl, cyclohexyl, ammonium, aminoethyl, and aminopropyl groups. POSS molecules have been used to modify various polymers, which are the subject of various patents. In the high temperature polymer area, for example, polyimides have been modified with aminopropylisobutyl POSS in Poe, et al. U.S. Application Publication Nos. 2009/0069508 (2008) and 2010/0063244 (2009). These types of moieties, in general, pose a thermal and solubility mismatch with polymers considered to be high temperature (largely aromatic). Disassociation of the POSS organic periphery can produce free radicals that may react with any polymer in the immediate vicinity, thus causing chain scission and property weakening. The sole commercial compound that possesses a thermally stable, homogeneous aniline periphery has the drawback of possessing a high density of reactive groups. When reactively is incorporated into a polymer, this characteristic generally contributes to either the formation of an over-crosslinked network resulting in material embrittlement or incomplete amine conversion (due to steric barriers resulting in sites that exhibit an affinity to water, which are deleterious toward material aging manifested in thermo-mechanical properties). Therefore, there exists a significant need for the development of new, thermally stabile, amino-functionalized POSS compounds with a lower concentration of reactive groups. Such molecules would be beneficial in the design and control of polymeric architectures for the purposes of maximizing delivered properties, especially at elevated temperatures. Further utility of the embodied aniline POSS compounds is readily derived from chemical reaction of their amine groups with anhydride compounds to form imide-type moieties with other types of reactive end-groups, notably phenylethynyl phthalic anhydride (PEPA) (a crosslinkable group found on the ends of many high performance thermosetting oligomeric compounds). Crosslinking proceeds during consumption of the ethynyl group through cyclotrimerization to form a thermally and mechanically robust aromatic junction.
The nomenclature for silicon atoms was originally developed to distinguish silicon monomers and polymers. A silane (M) group consists of three organic groups and one oxygen atom bound to a central silicon atom. A siloxane (D) group consists of two organic groups and two oxygen atoms bound to the central silicon atom. When one organic group and three oxygen atoms bound to a central silicon atom, the group is referred to as a silsesquioxane (T) group. A silicate (Q) group consists of four oxygen atoms bound to the central silicon atom, which is not bound to any organic groups. The structure of each group is illustrated below. This method allows the simplified description of various structures.
